Field emission devices (FEDs) or micro-vacuum tubes have gained recent popularity as alternatives to conventional semiconductor silicon devices. Advantages associated with the use of FEDs include faster switching in the terahertz regime, temperature and radiation insensitivity, and relative ease in fabrication. Applications range from discrete active devices to high density SRAMs and displays, radiation-hardened military applications, and temperature insensitive space technologies, for example. The literature on field emission devices focuses principally on process problems associated with producing the sharpest tip (e.g., with photolithography), controlling cathode to anode and cathode to gate distances, and achieving self-alignment between these elements. In many known vertical FEDs, the sharply pointed tip of the cathode is the only physical structure that is not commonly produced by standard integrated circuit fabrication processes.
Recently, lateral field emission devices have emerged as an alternative to traditional vertical emitter devices. U.S. Pat. Nos. 5,233,263 and 5,308,439 to Cronin et al. describe lateral field emission devices in which conventional integrated circuit fabrication techniques are used to produce the devices. A horizontal thin-film cathode is disclosed wherein the cathode emitter tip is separated from an anode by a predetermined distance. The anode receives electrons emitted horizontally by field emission from the tip of the cathode. For significant electron tunneling to take place at the tip of the emitter, the electric field at the tip must reach a relatively high strength (e.g., 1.times.10.sup.7 V/cm).
The electric field produced at the cathode tip determines the electron density emitted by the cathode which in turn determines the current of the device. The magnitude of the cathode electric field is partially controlled by varying the applied voltage to gates disposed above and/or below the cathode. Changes in the gate voltage will cause corresponding changes in the electric field. Because the device output current changes exponentially with changes in the cathode electric field, even small changes in the electric field strength will produce high gains in the device.
For a given gate voltage the strength of the electric field produced between the emitter tip and the anode can additionally be controlled by varying such geometric factors as the horizontal distance between the cathode emitter and the anode (gap spacing) and the vertical insulator spacing between the gate and the emitter. The smaller the gap between the emitter and the anode, the stronger the electric field produced. In addition, when the vertical distance between the cathode and the gate is minimized (i.e. when the thickness of the insulator separating the emitter and the gate is minimized), an electric field of increased strength will result for a given gate voltage.
Electrons emitted from the emitter tip will often be collected on parts of the gate electrode. Such collections increase in frequency with the positive voltage (with respect to the emitter electrode) that must be placed on the gate electrode to get the device to function. As a result, the number of electrons that reach the anode is reduced, and the efficiency and transconductance of the device is lowered. For a given applied voltage, Cronin et al. teaches in the aforementioned patents that the electric field can be further increased by terminating the cathode tip in the same vertical plane as the gate edge such that the vertical plane is orthogonal to the upper surface of the underlying substrate. Electron collisions with the gate electrode are therefore minimized. In addition, the smaller the radius of curvature of the projecting emitter tip, the lower will be the gate voltage necessary to initiate electron flow (threshold voltage).
In summary, the requisite emitter electric field of lateral field emission devices, and, thus, the current of the device is somewhat controllable by the gate voltage by optimizing the aforementioned parameters during fabrication. Additional control of device current through the design of a high gain structure is desirable. An optimized device structure is desired wherein the change in anode current for a given change in gate voltage is substantially increased, while maintaining acceptable current.